This application is based on application No. 11-159099 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a cathode-ray tube (CRT) system, which is a representative example of a display device.
(2) Description of the Prior Art
One fundamental requirement for a display device, such as a computer monitor, is to display small characters clearly in every part of the screen.
A CRT device used as a display device emits and deflects an electron beam so as to scan the entire viewing area. As the deflected beam travels further to reach positions away from the center of the screen, a spot formed by focusing the beam is likely to be distorted near the edges of the screen, so that the shape and size of spots differ at different positions in the screen. This usually results in differences in resolution between the center and outer areas of the screen, with the outer areas generally having a lower resolution.
Japanese Laid-Open Patent Application No. 7-327145 discloses a technique to provide uniform resolution across the viewing area of a CRT device. With this technology, a CRT device is provided with a deflection waveform generating circuit that raises the speed of the horizontal scanning at positions near the center of the screen and lowers it at positions near the edges of the screen. Video data is read according to a horizontal deflection signal generated by this deflection waveform generating circuit.
This technique increases the diameter of the beam spot near the center of the screen and reduces the diameter near the edges of the screen to counteract the stated phenomenon. This makes the spot diameter almost uniform across the screen, which also makes the resolution almost uniform across the screen.
In this way, the stated technique can change a diameter of a beam spot (hereafter a scanning spot) perceived by the viewer per unit time during the scanning. However, this technology cannot reduce a diameter of a xe2x80x9cstaticxe2x80x9d spot formed by the electron beam. Accordingly, when resolution should be enhanced further, manufacturers have to use other techniques to reduce the diameter of the static spot, such as by improving the performance of the electron gun.
In view of the above problems, the first object of the present invention is to provide a CRT system that can display images of a uniform and high resolution in every area of the screen without a complex construction being included in the CRT system.
The second object of the present invention is to reduce the diameter of a static spot without giving a special construction to an electron gun.
The third object of the invention is to provide a technique that reduces large diameters of scanning spots to a diameter of a small spot and makes every scanning spot diameter small and substantially the same.
The above objects can be achieved by a CRT system of a raster scan type that displays images on a screen by deflecting an electron beam, which has been modulated using a video signal, to consecutively scan the screen in a main scanning direction while scanning in a sub-scanning direction that is perpendicular to the main scanning direction, the CRT system including: a speed modulating unit for modifying a deflection speed by modulating a deflection signal that is used to deflect the electron beam in either the sub-scanning direction or the main scanning direction, the deflection speed being a speed at which the electron beam traverses the screen in either the sub-scanning direction or the main scanning direction; an amplitude modulating unit for performing amplitude modulation on the video signal; and a frequency modulating unit for performing frequency modulation on the deflection signal and/or the video signal either before or after the amplitude modulation.
For this construction, resolution becomes almost uniform across the screen. In addition, the diameter of a static spot can be reduced.
Here, the sub-scanning direction may be a vertical direction of the screen and the main scanning direction may be a horizontal direction of the screen. The speed modulating unit may modify the deflection speed to high at positions near a center of the screen and low at positions near edges of the screen, the deflection speed being a horizontal deflection speed at which the electron beam traverses the screen in the horizontal direction. The amplitude modulating unit and the frequency modulating unit may perform the amplitude modulation and the frequency modulation, respectively, on the video signal in synchronization with a horizontal scan cycle during which the electron beam makes one complete traverse of the screen in the horizontal direction.
With this construction, brightness increases near the right and left edges of the screen. This increase in brightness contributes to raising resolution near the edges of the screen.
Here, the amplitude modulating unit may perform the amplitude modulation on the video signal to raise an amplitude of the video signal-at the positions where the horizontal deflection speed is made high, and to lower the amplitude of the video signal at the positions where the horizontal deflection speed is made low. The frequency modulating unit may perform the frequency modulation on the video signal to raise a frequency of the video signal at the positions where the horizontal deflection speed is made high, and to lower the frequency of the video signal at the positions where the horizontal deflection speed is made low.
For this construction, resolution near the right and left edges of the screen can be enhanced without changing brightness of original images and distorting the images.
Here, the deflection signal may be a horizontal deflection signal, and the speed modulating unit may be a circuit that performs S-shaping modulation on the horizontal deflection signal to shape a wave of the horizontal deflection signal into an S-shape. The S-shaping modulation may change the horizontal deflection speed to a higher extent than is required to counteract differences in the horizontal deflection speed that are present at different positions of the screen, the differences resulting from differences in deflection sensitivity between: (a) right and left edges of the screen; and (b) a center part of the screen.
With this construction, it is possible to perform scanning on two scanning lines during each deflection cycle.
Here, a horizontal scan may be performed in a reciprocating manner in which scanning directions on adjacent horizontal scanning lines are opposite.
Here, the sub-scanning direction may be a vertical direction of the screen, and the main scanning direction may be a horizontal direction of the screen. The speed modulating unit may modify the deflection speed to high at positions near a center of the screen and low at positions near edges of the screen, the deflection speed being a vertical deflection speed at which the electron beam traverses the screen in the vertical direction. The amplitude modulating unit may perform the amplitude modulation on the video signal in synchronization with a vertical scan cycle in which the electron beam makes one complete traverse of the screen in the vertical direction. The frequency modulating unit may perform the frequency modulation on the video signal and a deflection signal that is a horizontal deflection signal in synchronization with the vertical scan cycle.
Here, the amplitude modulating unit may perform the amplitude modulation on the video signal to raise an amplitude of the video signal at the positions where the vertical deflection speed is made high, and to lower the amplitude of the video signal at the positions where the vertical deflection speed is made low. The frequency modulating unit may perform the frequency modulation on the video signal and the horizontal deflection signal to raise frequencies of the horizontal deflection signal and the video signal at the positions-where the vertical deflection speed is made high, and to lower the frequencies of the video signal and the horizontal deflection signal at the positions where the vertical deflection speed is made low.
The above objects can be also achieved by a CRT system of a raster scan type that displays images on a screen by deflecting an electron beam, which has been modulated using a video signal, to consecutively scan the screen in a main scanning direction while scanning in a sub-scanning direction that is perpendicular to the main scanning direction, the CRT system including: a first speed modulating unit for modifying a first deflection speed by modulating a first deflection signal in first cycles equal to a period during which the electron beam makes one complete traverse of the screen in the sub-scanning direction, the first deflection signal being used to deflect the electron beam in the sub-scanning direction, the first deflection speed being a speed at which the electron beam traverses the screen in the sub-scanning direction; a first amplitude modulating unit for performing first amplitude modulation on the video signal in the first cycles; a first frequency modulating unit for performing, in synchronization with the first amplitude modulation, first frequency modulation on: (a) the video signal; and (b) a second deflection signal that is used to deflect the electron beam in the main scanning direction, the first frequency modulation being performed either before or after the first amplitude modulation; a second speed modulating unit for modifying a second deflection speed by modulating the second deflection signal in second cycles equal to a period during which the electron beam makes one complete traverse of the screen in the main scanning direction, the second deflection speed being a speed at which the electron beam traverses the screen in the main scanning direction; and a second amplitude modulating unit for performing second amplitude modulation on the video signal in the second cycles; and a second frequency modulating unit for performing, in synchronization with the second amplitude modulation, the second frequency modulation on the video signal, the second frequency modulation being performed either before or after the second amplitude modulation.
Here, the sub-scanning direction may be a vertical direction of the screen and the main scanning direction may be a horizontal direction of the screen. The first speed modulating unit may modify the first deflection speed to high at positions near a center of the screen and low at positions near edges of the screen, the first deflection speed being a vertical deflection speed at which the electron beam traverses the screen in the vertical direction. The first amplitude modulating unit may perform the first amplitude modulation to raise an amplitude of the video signal at the positions where the vertical deflection speed is made high, and to lower the amplitude of the video signal at the positions where the vertical deflection speed is made low. The first frequency modulating unit may perform the first frequency modulation to raise frequencies of the second deflection signal and the video signal at the positions where the vertical deflection speed is made high, and to lower the frequencies of the second deflection signal and the video signal at the positions where the vertical deflection speed is made low. The second speed modulating unit may modify the second deflection speed to high at positions near a center of the screen and low at positions near edges of the screen, the second deflection speed being a horizontal deflection speed at which the electron beam traverses the screen in the horizontal direction. The second amplitude modulating unit may perform the second amplitude modulation to raise an amplitude of the video signal at the positions where the horizontal deflection speed is made high, and to lower the amplitude of the video signal at the positions where the horizontal deflection speed is made low. The second frequency modulating unit may perform the second frequency modulation to raise a frequency of the video signal at the positions where the horizontal deflection speed is made high, and to lower the frequency of the video signal at the positions where the horizontal deflection speed is made low.